Lecture 13 - RNA Processing Flashcards
What are the 3 processing events mRNA undergoes to become mature?
3 processing events the mRNAs in eukaryotes will undergo - particularly in higher eukaryotes genes are divided into exons and introns:
At 5’ end production of the cap
Cleavage at polyA site - addition of polyA tail is lots of A residues that make up the 3’ end of most mRNAs
Splicing - the removal of introns.
Capping occurs very early in transcription
These events coordinated by RNA pol II
How does capping take place?
3 enzymes act in succession
phosphatase
guanyl transferase
methyltransferase
Modification that occurs at the 5’ end of the transcript
Phosphatase removes a phosphate group from the original 5’ end of the mRNA
Guanyl transferase adds a guanosine residue to the 5’ end - does this in unusual way - in RNA normally the linkage in the phosphodiester backbone is between the 5’ and 3’ carbons - the cap is put on in a “reverse linkage” linking 5’ to 5’. This is significant because most of the enzymes that chew up RNA chew up 5’-3’ linkages so this adds additional flexibility - only certain enzymes can break off the cap. The guanosine is then methylated by methyltransferase
Newly formed pre-mRNA is capped early ~ after 25-30 bases
Capping enzymes bind to phosphorylated RNAP II CTD
What proteins are involved in cleavage and polyadenylation?
Cleavage stimulation factor F (CstF)
Cleavage and polyadenylation specificity factor (CPSF)
Travel with the phosphorylated CTD tail of RNA polymerase II during transcription
Transfer to the 3’ end processing sequences on the RNA molecule once the sequences are transcribed
How is splicing carried out?
Splicing is performed by the spliceosome - a highly complex molecular machine composed of small nuclear ribonucleoprotein particles (snRNPs “snurps”)
What is the proteome?
the entire set of proteins expressed in a cell/tissue/organism at a certain time
What is the transcriptome?
the entire set of RNA molecules expressed in a cell/tissue/organism at a certain time
